Grenade launchers attachable to rifle barrels known in the prior art usually involve spring mechanisms to counteract the launching force. Springs are poorly suited to this purpose where a maximum of acceleration is desired in the launching phase of operation, since compression of the spring may oppose such acceleration. Also, when two or more grenades are launched in rapid succession, it is desirable that the launcher return to the ready position rapidly after each shot, and with the least possible recoil force applied through the rifle stock. In addition, springs are well known to change their characteristics during prolonged use, especially where shock loads are involved, hence any system designed for a certain narrow range of spring forces will soon be operating outside the design condition because of the mentioned change in spring characteristics.